With Gauge Data in the Amazon Basin
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VALIDATION OF THE LAND WATER STORAGE FROM GRAVITY RECOVERY AND CLIMATE EXPERIMENT (GRACE) WITH GAUGE DATA IN THE AMAZON BASIN Validação da avaliação pelo GRACE do acúmulo de água no subsolo da bacia amazônica com base em dados de linígrafos 1 SONIA MARIA ALVES COSTA 2 ANA CRISTINA OLIVEIRA CANCORO DE MATOS 2 DENIZAR BLITZKOW 1Instituto Brasileiro de Geografia e Estatística Diretoria de Geociências, Coordenação de Geodésia Av. Brasil, nº 15.671 Bloco IIIA sala 06 Parada de Lucas - Rio de Janeiro – RJ CEP 21241-051 [email protected] 2Escola Politécnica da Universidade de São Paulo- EPUSP-PTR Laboratório de Topografia e Geodésia Caixa Postal 61548, CEP: 05424-970, São Paulo, São Paulo, Brasil [email protected]; [email protected] ABSTRACT The Amazon basin is a region of constant scientific interest due to its environmental importance and its biodiversity and climate on a global scale. The seasonal variations in water volume are one of the examples of topics studied nowadays. In general, the variations in river levels depend primarily on the climate and physics characteristics of the corresponding basins. The main factor which influences the water level in the Amazon Basin is the intensive rainfall over this region as a consequence of the humidity of the tropical climate. Unfortunately, the Amazon basin is an area with lack of water level information due to difficulties in access for local operations. The purpose of this study is to compare and evaluate the Equivalent Water Height (Ewh) from GRACE (Gravity Recovery And Climate Experiment) mission, to study the connection between water loading and vertical variations of the crust due to the hydrologic. In order to achieve this goal, the Ewh is compared with in-situ information from limnimeter. For the analysis it was Bol. Ciênc. Geod., sec. Artigos, Curitiba, v. 18, no 2, p.262-281, abr-jun, 2012. Costa, S. M. A. et al. 263 computed the correlation coefficients, phase and amplitude of GRACE Ewh solutions and in-situ data, as well as the timing of periods of drought in different parts of the basin. The results indicated that vertical variations of the lithosphere due to water mass loading could reach 7 to 5 cm per year, in the sedimentary and flooded areas of the region, where water level variations can reach 10 to 8 m. Keywords: GPS; GRACE; Limnimeter Stations. RESUMO A bacia Amazônica é uma região de constante interesse científico devido à sua importância ambiental e climática e à sua biodiversidade em uma escala global. As variações sazonais do volume d’água é um dos exemplos de temas estudados atualmente. Em geral, as variações dos níveis dos rios dependem basicamente do clima e das características físicas das bacias hidrográficas correspondentes. O fator principal de grande influência no nível d’água na bacia Amazônica são as elevadas precipitações na região em conseqüência da umidade do clima tropical. Infelizmente a bacia Amazônica é uma região com carência de informações de dados do nível da altura d’água devido a dificuldade de acesso para operações locais. A proposta deste trabalho é comparar e avaliar as grades do Equivalente à altura d’água (Ewh) da missão GRACE (Gravity Recovery And Climate Experiment), para estudar o ciclo hidrológico e sua relação com a variação da litosfera na região. Com este propósito os dados GRACE (Ewh) são comparados com observações in-situ obtidas dos linígrafos. Para esta validação foram calculados o coeficiente de correlação, fase, amplitude do GRACE (Ewh) e dos dados in-situ, bem como, os períodos de seca em diferentes partes da bacia. Os resultados indicam que as variações verticais da litosfera devido à carga da massa d’água podem chegar de 7 a 5 cm por ano nas áreas sedimentares e alagadas da região, onde as variações do nível d’água podem alcançar de 10 a 8 m. Palavras-chave: GPS; GRACE; Estações Linimétricas. 1. INTRODUCTION The water surface knowledge has several applications, such as monitoring and forecasting flood, sediment flows over basins and geomorphology domains. In general, the limnimeters are very sparsely located or do not exist in a sufficient number to provide a detailed evaluation of the river basin; this is the case of the Amazon. This one is the greatest river basin in the world due to its extension and to the strong seasonal water level variation, reaching 10 m near Manaus against 4 m in regions in the south, near to Alta Floresta and in the north near to Boa Vista (Figure 1). Nowadays, with the advances of satellite technology, it is possible to monitor any information regarding hydrology, lithosphere, geomorphology and climate in a global and regional context important for ecosystems evaluation and its monitoring. A number of studies have exploited the water storage variability on land, more Bol. Ciênc. Geod., sec. Artigos, Curitiba, v. 18, no 2, p.262-281, abr-jun, 2012. 264 Validation of the land water storage from gravity reconvery and... specifically for continental hydrology (RAMILLIEN et al., 2005; CHEN et al., 2005; CHAMBERS et al., 2006; TAPLEY et al., 2004a; WAHR et al., 2004). The water storage estimates used in these studies are typically presented as spatial averages over regions having scales of a few hundred km and greater. ALMEIDA (2009) concentrated his study in the Amazon region, validating GRACE Ewh with water level in-situ data, but did not explore the interaction of hydrosphere over lithosphere. Considering that GRACE mission provides the gravity field variations, it means that the temporal geoid models through Stokes coefficients can be derived and an inversion procedure applied, from which the Equivalent Water Height (Ewh) anomalies are obtained. The Ewh expresses the variation of water mass storage including soil moisture, ice sheets, glaciers, oceans, etc. The objective of this study is to evaluate GRACE grids from Ewh anomalies with in-situ data from limnimeter and permanent GPS stations, respectively, in order to analyze grids agreement with in-situ water level observations and estimate their confidence when used for the evaluation of the water cycle and its connection with the lithosphere displacement. The in-situ or local lithosphere vertical displacement monitored by the coordinate time series of a GPS reference network established in the Brazilian part of the Amazon region. The data available are provided by three different projects and institutions: Brazilian Network for Continuous Monitoring of GNSS (RBMC), Low-Latitude Ionospheric Sensor Network (LISN) and Serviço Geológico Brasileiro (CPRM). A total of 24 stations were used, 20 of them in the Amazon Basin and four in neighbor basins, Paraná and Tocantins. The in-situ water level time series comes from limnimeter stations of Agência Nacional de Águas (ANA). A total of 84 limnimeter stations were selected in a radius of 100 km around the GPS stations. The water level variation time series of these stations were used for the comparison and analysis with Ewh grids. The time series of geodetic heights variation are used to estimate the anti-correlation with linmimeters data. The GRACE grids came from the second release (RL02) of 10-day vertically- integrated Ewh grids inferred from GRACE gravity field models (10-day models). They were computed by the Centre National d’Etudes Spatiales/Groupe de Recherches de Géodésie Spatiale (CNES/GRGS) and they represent the information about the total water storage variations that includes ground and surface water and snow packs (FRAPPART et al, 2011). The analysis accomplishes four years of data, from January 2007 to December 2010, when a reasonable number of GPS stations were established in the Amazon basin and two neighbor basins, Tocantins and Paraná. The distribution of GPS and limnimeter stations, as well as basins and sub- basins division are presented in Figure 1. For a better understanding, this paper is divided into 7 parts. Section 2 presents some important aspects of the study region. Section 3 describes GPS data processing and presents the results. The time series construction of in-situ datasets (limnimeter and GPS) is presented in section 4. The methodology and particularities of the solution strategy of CNES/GRGS Ewh grids is presented in section 5. The Bol. Ciênc. Geod., sec. Artigos, Curitiba, v. 18, no 2, p.262-281, abr-jun, 2012. Costa, S. M. A. et al. 265 Ewh anomaly time series computed from the limnimeter stations coordinates in Amazon and neighbor basins are compared with limnimeter time series and the correlation, amplitude and phase results were evaluated in section 6. The summary and conclusions are presented in section 7. Figure 1 - Distribution of GPS stations (red), limnimeter stations (green) in basins and sub-basins division. 2. THE STUDY REGION – AMAZON BASIN AND NEIGHBOR BASINS The Amazon Basin covers 6,110,000,000 m² (ANA, 2009) of the South American continent and it has the largest water volume in a continental area. The Brazilian Amazon is one of the most humid areas in the world. The main factor which influences the water level in the Amazon Basin is the intensive rainfall as a consequence of the humidity of the tropical climate. A considerable part of the rain in the Amazon basin is supplied by the ecosystems evapotranspiration, with an annual average contribution from 55% to 60% from the total precipitation (GUIMARÃES et al., 2012). It is a region with a constant scientific interest due to its environmental and climate importance on a global scale and its biodiversity. The great seasonal variation of the water volume and the great amount of sediments transported by the rivers are examples of scientific fields investigated nowadays (MARENGO et al., 2008; MARENGO et al., 2011). Generally, river level variations depend basically on the climate and physics characteristics of the corresponding basin.